a) Expand the landscape

The new tile must touch some already-placed tile, but there are no restrictions about what types of terrain are adjacent to each other.

The first player's first move of course is an exception, since there is no already-placed tile. The first player's first move is to simply place their tile on the table (and then place a hut on one of its non-volcano hexes).

b) Volcanic eruption

The volcano hex on the new tile must be placed directly on a volcano hex already in play.

The volcano tile (which has 3 hexes) must be on top of at least 2 tiles.

All three hexes under the eruption tile must be at the same level.
http://www.boardgamegeek.com/thread/262030

Huts lost to volcanic eruptions leave the game and cannot be reused. They still count as having been placed (for purposes of ending the game by building all of two types of your buildings).
http://www.boardgamegeek.com/thread/155695
http://www.boardgamegeek.com/thread/164250

It is perfectly legal (and sometimes good for you) to cover your own huts.

It is legal to cover volcanoes with the non-volcano hexes of the new tile.
http://www.boardgamegeek.com/thread/198540

It is illegal to cover a temple or tower.

It is illegal to cover all of a one-hut or two-hut settlement, wiping out the entire settlement.

There is no limit to the number of levels you can build. E.g. the rules specifically say Towers can be built on "a level 3 or higher field".
http://www.boardgamegeek.com/thread/175258

a) Build a Hut

A single hut can be placed on any empty level 1 hex. You cannot use this action to place a single hut on a higher level.

You can use this action to place a hut adjacent to an existing settlement. This not considered an expansion, but simply building a hut.

The first player places a building (which must be a hut) on their first turn, just like every other turn.
http://www.boardgamegeek.com/thread/273939

b) Build a Tower

The tower must be placed on level 3 or higher, adjacent to one of your settlements. It doesn't matter if this settlement has temples or huts or both, nor how many hexes it occupies. All that matters is that it's next to an empty level hex where you can put the tower, and that the settlement doesn't already have a tower.

A Tower could be built next to a Hut or a Temple.

If a space is next to a settlement with a tower and another settlement without a tower, you may choose the latter settlement to legally build the tower from.

c) Build a Temple

Levels have no relevance. All that matters is that the settlement doesn't already have a temple, and that it covers at least 3 hexes. One of those hexes could be a tower.

A Temple could be build next to a Hut or a Tower.

If a space is next to a settlement with a temple and another settlement without a temple, you may choose the latter settlement to legally build the temple from.

d) Expand an existing Settlement

You name a terrain type that is adjacent to the settlement, and must fill all empty hexes of the named terrain type with huts. Note that you may only fill hexes that are directly adjacent to the existing settlement (i.e. you cannot cascade hut placement to other hexes that are not directly adjacent). Empty hexes on level one are filled with one hut, empty hexes on level two are filled with two huts, etc. If you don't have enough huts left, then the entire move is illegal, and you cannot do it. The rules explicitly say of the building actions that "a player may only choose one for which he still has enough building pieces"
http://www.boardgamegeek.com/article/1468964#1468964
http://www.boardgamegeek.com/article/1205048#1205048

It is perfectly possible that your expansion causes you to build huts in only one hex, if that is the only hex of the named terrain type adjacent to your settlement. And if that hex is at level 1, then you place only 1 hut. (That case is in effect indistinguishable from the action "Build a Hut".)